Method and apparatus for control of switched reluctance motors

ABSTRACT

A short pitched switched reluctance motor control apparatus comprising a voltage provider comprising a first coupling and a second coupling configured to be coupled to a phase winding of the switched reluctance motor for applying a voltage to drive current in the winding between the first and second coupling is disclosed. The apparatus further comprises a controller configured to apply a first voltage pulse to the first coupling, and to apply a second voltage pulse to the second coupling, wherein the start of the second pulse is delayed with respect to the start of the first pulse, and the end of the first pulse is delayed with respect to the end of the second pulse.

RELATED APPLICATIONS

This application is a National Phase of PCT Patent Application No.PCT/GB2014/053356 having International filing date of Nov. 12, 2014,which claims the benefit of priority of United Kingdom PatentApplication No. 1319967.4 filed on Nov. 12, 2013. The contents of theabove applications are all incorporated by reference as if fully setforth herein in their entirety.

The present disclosure relates to the control of switched reluctanceelectric motors, and more particularly to apparatus and methods forcontrolling the power supply to switched reluctance motors, and stillmore particularly to controlling the power supply to short pitchedswitched reluctance motors.

Switched reluctance, SR, motors use field coils wound onto a stator anda solid salient-pole rotor made of soft magnetic material (such aslaminated-steel). The rotor does not generally carry any windings. Thestator windings are arranged on angularly separated teeth. When power isapplied to a stator winding, the rotor's magnetic reluctance tends toalign the rotor pole with the tooth (magnetic pole) of the stator thatcarries that winding. In order to rotate the rotor, the windings ofsuccessive stator poles can be energised in sequence so that themagnetic field of the stator “leads” the rotor pole to rotate.

SR motors can provide significant advantages. They are robust and lowcost due to simple stator windings and geometry. They cannot however beconnected direct to an AC supply. The power electronics and controlneeded to drive an SR motor are a significant cost, and it has generallybeen thought that the way to reduce this cost is to use powerelectronics having a lower number of switches per phase.

SUMMARY OF THE INVENTION

Embodiments of the present disclosure relate to SR motors, and theircontrol. Different types of SR motors exist. SR motors may be “fullypitched”, or “short pitched”. In a “short pitched” SR motor each statortooth has a coil wrapped around it. For example, in a three phase shortpitched SR motor the coil on every third stator tooth is connectedtogether, either in series or in parallel, to form one phase winding. Ina “short pitched” SR motor the torque is generated from theself-inductance of the windings. The effects of the self-inductance ofeach phase winding dominate (e.g. are much greater than) the mutualinductance between phase windings. In “fully pitched” SR motors eachwinding is distributed between the stator teeth, and may be wound on asmany stator teeth as there are phases in the power supply. As a resultthere is flux coupling between the windings of different phases, and thevariation in mutual coupling between two excited phases of the motor isused to provide torque in the rotor.

The torque developed in an SR motor may be expressed as follows:

$\begin{matrix}{T = {{\frac{1}{2}i_{a}^{2}\frac{\mathbb{d}L_{a}}{\mathbb{d}\theta}} + {\frac{1}{2}i_{b}^{2}\frac{\mathbb{d}L_{b}}{\mathbb{d}\theta}} + {\frac{1}{2}i_{c}^{2}\frac{\mathbb{d}L_{c}}{\mathbb{d}\theta}} + {i_{a}i_{b}\frac{\mathbb{d}M_{ab}}{\mathbb{d}\theta}} + {i_{b}i_{c}\frac{\mathbb{d}M_{{bc}\;}}{\mathbb{d}\theta}} + {i_{c}i_{a}\frac{\mathbb{d}M_{ca}}{\mathbb{d}\theta}}}} & {{Equation}\mspace{14mu}(1)}\end{matrix}$

In Equation 1 the subscripts (a, b, c) indicate the first second andthird phases respectively. L_(a), L_(b), L_(c) indicate theself-inductance of each phase of the windings, and M_(ab), M_(bc),M_(ca) indicates the mutual inductance of the phases indicated by thesubscripts.

In a short pitched SR motor, the self-inductance terms dominate (L>>M),so the torque does not depend on the direction (sign) of the current. Bycontrast, in fully pitched SR motors, the mutual inductances dominate,and the control of the relative directions (signs) of the currentsdetermine the torque.

There exists a prejudice in the art that, as the direction of current isunimportant in a short pitched SR motor, to reduce the cost of drivecircuitry it is best to use asymmetric bridges in which lower costdiodes carry the current in the “freewheel” period, and current flows inone direction through each phase winding. This reduces the number of(more expensive) power transistors that need to be used. When an SRmotor is stationary and required to produce large torque starting fromrest, in a conventional, asymmetric, SR drive the switching devices(such as controllable impedances, e.g. power transistors) of the driveonly carry current for a short time whilst the diodes carry the currentalmost all of the time.

By contrast, embodiments of the present disclosure aim to reduce thepower burden carried by the switching devices in a controller for ashort pitched SR motor. This may enable the use of switching deviceswith lower power tolerance and/or may extend the working life ofswitching devices.

Embodiments of the disclosure employ a symmetric bridge, in which, underthe same conditions, the direction of the voltage bias applied by thedrive to the phase winding can be reversed between cycles of the motor.Accordingly, the current can be shared out between a greater number ofdevices so the average current per device can be reduced.

This may be achieved by balancing the thermal load carried by eachdevice, for example the switching cycles may be chosen so that asimilar, for example approximately equal or equal, amount of power isdissipated by each device when averaged over a large number of cycles.This can enable devices with a lower current rating to be used. Inaddition, because power loss can be shared more equally amongst powerdevices this may improve heat distribution in the drive and facilitatecooling.

One embodiment which aims to achieve this comprises a motor controlleradapted to control the current in a phase winding of a short pitched SRmotor. This motor controller comprises a voltage provider comprisingcontrolled impedances arranged to provide an H-Bridge. The voltageprovider is configured to be coupled to the phase winding and operableto selectively apply voltage to the winding to drive current in thephase winding in a first direction and in a second direction opposite tothe first direction. A controller of the voltage provider is configuredso the direction of the applied voltage alternates between the firstdirection and the second direction. This alternation may take place overone or more successive cycles of the motor, and the timing of thereversals may be selected so that, over a large number of cycles, thethermal load in the bridge is evenly shared between the controlledimpedances (for example the power dissipated in each controlledimpedance of the H-bridge) may be equal, for example the controller maybe adapted to reduce the time average of the current through the windingby balancing negative and positive (first and second direction) currentsthrough the winding.

When a SR motor is operating over the speed and torque range bothmotoring and generating the duty cycle and current profile varies. In aconventional SR machine the switch devices and didoes have duty cycleswhich cover a wide range. These devices therefore have to be rated forthe worst case operating points. Embodiments of the disclosure employbipolar (e.g. bidirectional) currents in the phase windings of themotor. Accordingly, the average current can be distributed more evenlybetween switching devices (such as controlled impedances) in the drive,and the worst case average current per device is reduced and so deviceswith a lower current rating can be used.

Embodiments of the disclosure use centre aligned PWM to reduce theenergy loss in each switching device when the motor is at low or zerospeed. The benefit of this may reduce as motor speed increases, howeverin some embodiments the direction of the current in a phase winding canbe alternated between cycles and this distributes the losses at greaterspeed as the benefits of using centre aligned PWM are reduced.Therefore, the combination of centre aligned PWM (which need not besymmetric) and the use of bidirectional currents, provides a synergisticbenefit.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Embodiments of the disclosure will now be described, by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 shows a schematic diagram of an apparatus comprising an SR motor;

FIG. 2 shows a symmetric bridge for supplying power to a phase windingof an switched reluctance motor;

FIG. 3A shows a pulse width modulation scheme;

FIG. 3B shows a pulse width modulated signal provided by the scheme ofFIG. 3A; and

FIG. 4 shows a schematic circuit diagram of electronics for use in theapparatus shown in FIG. 1;

DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION

FIG. 1 shows an apparatus comprising a short pitched SR motor 1, and acontrol apparatus.

The SR motor 1 comprises a six pole stator, and a four pole rotor 6. Thepoles of the stator carry inductive windings, and the windings onopposing poles are coupled together in series.

The control apparatus comprises a controller 4 and three voltageproviders. Each voltage provider 2 is coupled to one of the pairs ofinductive windings carried on an opposing pair of poles of the stator.Each voltage provider 2 comprises a first output coupling and a secondoutput coupling. Each opposing pair of stator windings is coupledbetween the first and second output coupling of a corresponding one ofthe voltage providers.

An encoder is coupled to the rotor 6, and an output of the encoder iscoupled to the controller 4. The encoder is operable to sense theorientation of the rotor 6 and to provide a signal to the controller 4indicating the sensed orientation.

The voltage providers are operable to apply a bidirectional voltage biasto the inductive windings to which they are each coupled, so that sothat each voltage provider 2 is operable to control, and to reverse, theflow of current in one phase of the switched reluctance motor 1 byapplication of voltage to the winding. The controller 4 is configured tocontrol the voltage providers to drive current, and change itsdirection, in the windings. The controller 4 is also arranged to obtainan indication of the orientation of the rotor 6 from the encoder, and tocontrol the current in the phase windings of the SR motor 1 based on thesensed orientation.

In operation, when the rotor 6 is rotating, the controller 4 controlsthe voltage providers based on the encoder signal so that the fluxapplied by the stators rotates between the phases in a manner which“leads” the rotor 6 in rotation.

To control the power provided to each winding, the controller 4 usespulse width modulation, PWM. To provide the pulses the controller 4controls the first output coupling of one of the voltage providers tochange the voltage of its first output coupling from a first voltage toa second voltage. After a selected delay, the controller 4 controls thesecond output coupling of that voltage provider 2 to change from thefirst voltage to the second voltage. Accordingly, during the delay,there is a voltage difference between the output couplings of thevoltage provider 2. The controller 4 then controls the second outputcoupling to change back from the second voltage to the first voltage.After another selected delay, the controller 4 controls the first outputcoupling to change back from the second voltage to the first voltage.Again, during this delay, there is a voltage difference between theoutput couplings of the voltage provider 2. The voltage differenceduring these delays provides a pulse of current in the phase windingcoupled to that voltage provider 2. If the two selected delays areequal, or approximately equal in length, this may be referred to ascenter aligned PWM.

When the rotor 6 is rotating, the controller 4 can control the voltageproviders each to provide a series of pulses. The integral of each pulseis selected so that the power in the phase winding varies with time in amanner which is synchronised with the rotation of the rotor. Thecontroller 4 modulates the pulses so the flux in each phase is angularlyoffset from the adjacent phase by π/3.

Details of each voltage provider 2 and the supply to a single phase ofan SR motor 1 will now be described FIG. 2.

FIG. 2 shows an apparatus comprising a voltage provider 2 and a singlephase winding L2 of an SR motor 1. The voltage provider 2 comprises afirst leg 10, having a first output coupling A1, and a second leg 12having a second output coupling A2. The first leg 10 and the second leg12 are coupled in parallel between a positive supply voltage 14, and anegative supply voltage 16.

The first leg 10 comprises a first controlled impedance M3 and a secondcontrolled impedance, M5. Each controlled impedance comprises aconduction path, and a control coupling. The conduction paths of thefirst and second controlled impedances M3, M5, are coupled together inseries between the positive supply voltage 14, and a negative supplyvoltage 16. The control couplings C3 C5, of the controlled impedancesM3, M5, are couplable to a controller 4 such as the controller 4 of FIG.1 (not shown in FIG. 2). The first leg of the voltage provider 2comprises a first output coupling arranged in series between theconduction paths of the first and second controlled impedances.

Similarly, the second leg 12 of the voltage provider 2 comprises a thirdcontrolled impedance, M4 and a fourth controlled impedance, M6. Thesetwo controlled impedances are coupled together in series, and thecontrol couplings of these controlled impedances C4, C6, are couplableto a controller 4 such as the controller 4 of FIG. 1 (not shown in FIG.2). A second output coupling, A2, is coupled in series between theconduction paths of the third controlled impedance and the fourthcontrolled impedance.

The phase winding L2 of the SR motor 1 is coupled between the firstoutput coupling and the second output coupling of the voltage provider2.

Each of the controlled impedances is operable to be switched into aconducting state, or into a non-conducting state by the application of acontrol signal to its control coupling.

The first leg is operable to control the voltage of the first outputcoupling based on control signals applied to the control couplings ofthe controlled impedances. The second leg is operable to control thevoltage of the second output coupling based on control signals appliedto the control couplings of the controlled impedances. By switchingthese impedances between conducting and non-conducting states, thevoltages of the output couplings can be switched to enable current to bepushed or pulled in either direction through the phase winding of the SRmotor 1.

In operation, the control signal provided to the second controlledimpedance is the inverse of the control signal provided to the firstcontrolled impedance. Likewise, the control signal provided to thesecond controlled impedance is the inverse of the control signalprovided to the first controlled impedance. The controller 4 switchesthe controlled impedances M3, M4, M5, M6, between conducting andnon-conducting (low impedance and high impedance states) thereby usingthe high and low supply voltages 14, 16 to control the voltage appliedto the phase winding L2. The difference between the voltage at the firstoutput coupling and the second output coupling applies a voltage bias tothe phase winding.

As illustrated in FIG. 3A, below, by driving the first coupling, A1,high, and after a first delay, driving the second output coupling, A2,high, the controller 4 applies a voltage pulse, A1-A2, to the phasewinding L2 for the duration of this first delay. The controller thendrives the second output coupling low and, after a second delay, drivesthe second output coupling low. Again, for the duration of this seconddelay a voltage pulse, A1-A2, is applied to the phase winding L2.Controlling the delays between the rise and fall of the first and secondoutput coupling respectively controls the width (duration) of thevoltage pulses applied to the phase winding. This enables center alignedpulse width modulation to be applied to control the voltage applied tothe phase winding L2.

This operation of a single phase of a short pitched SR motor 1 (e.g. asshown in FIG. 2) will now be described with reference to FIG. 2 and FIG.3A and FIG. 3B which shows a very schematic timing diagram of thesignals present in the circuit illustrated in FIG. 2.

FIG. 3A shows a plot of voltage against time plot indicating threevoltage traces. These three traces indicate the modulation of pulsesapplied to the phase winding shown in FIG. 2 during two time slices. Aplot of power against time for one phase winding of the SR motor 1 isshown in FIG. 3B.

Referring now to FIG. 3A, the first trace shows voltage variations overtime at the first output coupling, A1, of the voltage provider 2illustrated in FIG. 2. The second trace shows voltage variations overtime at the second output coupling, A2, of the voltage provider 2illustrated in FIG. 2. The third trace shows the variation of voltageapplied to the phase winding over time by the voltage difference A1-A2,between the first output coping and the second output coupling. Varyingthe time integral of this voltage difference in turn varies the powerapplied to the phase winding. Accordingly, by modulating the relativewidths of the pulses at A1 and A2, the length of the resultant twopulses applied to the phase winding, A1-A2, can be varied to control thepower transferred to the phase winding.

FIG. 3B shows a plot of current against time for a phase winding of theshort pitched SR motor shown in FIG. 1. FIG. 3B schematicallyillustrates two successive cycles of the SR motor. In the first cyclethe current is applied in a positive sense (corresponding to a flow ofcurrent in a first direction in the phase winding). During a secondcycle, the current is applied in a negative sense (corresponding to aflow of current in a second direction, opposite to the first direction,in the phase winding). As illustrated, each cycle of the SR motor maycomprise a large number of the time slices of the PWM cycle shown inFIG. 3A. However, in some implementations a single cycle of the motormay comprise only a single one of the time slices (e.g. a single PWMcycle) illustrated in FIG. 3A.

The controller 4 is configured to provide current in the phase windingin a first direction for a number of cycles of the motor 1, and then toreverse the direction of the current in the winding for a number ofcycles. The direction of the current may be reversed every cycle, or itmay be reversed less frequently, the reversals may be periodic, orintermittent, or triggered by an operational condition of the voltageprovider 2 or the motor 1, for example based on an encoder measurement,or heat sensed in the voltage provider 2.

As will be appreciated by the skilled addressee in the context of thepresent disclosure, the voltage providers 2, 2′, 2″ illustrated in FIG.1 may each comprise a voltage provider such as that illustrated in FIG.2. The controller 4 of FIG. 1 may be configured to control the directionof current in one phase winding of the short pitched switched reluctancemotor based on the direction of current in another one of the phasewindings. For example, although the mutual inductance terms, M, inEquation 1 are generally very small with respect to the self-inductance,L, of each winding, the controller may be configured to use currentreversals to control the sign of the applied currents in these windings.This may enable the controller to make selected adjustments in thetorque produced by the motor. For example, the controller may beconfigured to reverse the current in one or more of the windings with atiming selected to reduce torque ripple in the motor. For example, thereversals of current in the phase windings may be selected to coincidewith a frequency of the torque ripple, or a sub-harmonic, or a harmonicof the torque ripple. In some embodiments the timing of the reversalsmay be selected to increase the torque output from the motor, forexample so that the contribution of mutual inductance to the torquecombines additively with the contribution of the self-inductance. Thecontroller may also be configured to apply one or more currentreversals, or to select the frequency of current reversals based ondetecting a magnetic saturation condition in the motor.

As will be appreciated by the skilled addressee in the context of thepresent disclosure, the energy losses in a voltage provider 2 such asthe voltage provider 2 shown in FIG. 2 may be attributed both to ohmiclosses (associated with conduction) and losses associated with changingthe impedance of the controlled impedances (so called switching losses).As can be seen in FIG. 3, using a centre aligned PWM approach means thatthe switching frequency of the voltage applied to the phase winding istwice the switching frequency of the controlled impedances. Because themotor 1 is subjected to a higher frequency pulse train, with reducedenergy in each pulse, the torque ripple in the motor 1 and the acousticnoise generated by the motor 1 may both be reduced. Advantageouslyhowever this reduction may be achieved without increasing the switchinglosses that would otherwise be associated with increasing the frequencyof the pulses in the PWM pulse train.

FIG. 4 shows another possible implementation of a circuit for use in theapparatus described above with reference to FIG. 1. As described above,each voltage provider 2 shown in FIG. 1 may comprise a bridge havingfour controlled impedances, for example an H-bridge. In some exampleshowever, the controlled impedances of a three phase bridge may be sharedbetween the three phase windings of an SR motor 1.

The apparatus of FIG. 4 comprises a controller 4, a three legged bridge22, and a three phase short pitched SR motor 1. The SR motor 1 comprisesthree pairs of windings L1, L2, and L3, each pair corresponding to onephase. The controller 4 has three outputs, P1, P2, and P3.

Each leg of the bridge comprises first and second controlled impedances.Each controlled impedance comprises a conduction path, and a controlcoupling. The conduction paths of the first and second controlledimpedances are connected together in series between a positive supplyvoltage, and a negative supply voltage. The control coupling of thefirst controlled impedance of the first leg is coupled to a first one ofthe controller 4 outputs, P1. The controller 4 output P1 is coupled tothe control coupling of the second controlled impedance of the first legby a voltage inverter (e.g. a NOT gate). The controlled impedances ofthe second and third legs are coupled in the same way to the second andthird controller 4 outputs respectively.

The electrical points between the first and second controlled impedancesof each leg provide three outputs A1, A2, A3, from the bridge. The firstphase L1 of the SR motor 1 is coupled between the outputs A1, A2, of thefirst and second legs of the bridge. The second phase L2 of the SR motor1 is coupled between the outputs A2, A3, of the second and third legs ofthe bridge. The third phase L3 of the SR motor 1 is coupled between theoutputs A3, A1, of the third and first legs of the bridge. In operation,the controller 4 controls the outputs A1, A2 of the first and secondlegs of the bridge to provide the first and second voltage couplings ofa first voltage provider 2 as described above with reference to FIG. 1.Similarly, the controller 4 controls the outputs A2, A3 of the secondand third legs of the bridge to provide the first and second voltagecouplings of a first voltage provider 2 as described above withreference to FIG. 1.

Other alternatives and variations of the examples described above willbe apparent to the skilled addressee in the context of the presentdisclosure. For example the apparatus of FIG. 1 comprises an SR motor 1having a six pole stator, and a four pole rotor 6 clearly this is merelyexemplary, and motors and controllers having more or fewer poles andphases may be used.

The windings of each phase are described as being coupled together inseries, however they may also be coupled in parallel. The phase windingsmay consist solely of passive components without the use of any activedevices, such as diodes, to control the direction of current flow in thewindings. The phase windings have been described as being carried by thestator, however in some embodiments the windings may be arranged on therotor.

Although the apparatus of FIG. 1 is described as including an encoderthis is optional. For example, while flux is being applied by one phase,the controller may be configured to use the inactive phase windings tosense the orientation of the rotor—e.g. based on the inductance in thosephase windings. Other types of sensorless operation may be used. If anorientation sensor is used, it need not be an encoder, any type oforientation sensor may be used.

Controllable combination of current sources and/or current sinks may bearranged to push and/or pull current through the phase windings of an SRmotor 1. H-bridges, including four controlled impedances per inductivewinding may be used, or bridges including greater or lesser numbers ofcontrolled impedances. Other methods and apparatus of current controlmay be used. The term “symmetric bridge” should not be taken to implythat the bridge is geometrically symmetric.

The term center aligned PWM may comprise any scheme in which two pulsesof differing duration are additively combined and the start of theshorter of the two pulses is delayed with respect to the start of thelonger pulse (e.g. to provide a voltage difference to be applied to aphase winding as in FIG. 1, and the voltage A1-A2 in FIG. 3A). The startand end of the two pulses are delayed with respect to each other, butthese delays may be longer at the start than at the end of the pulse, sothe alignment of the pulses need not be symmetric, e.g. the centers ofthe two pulses need not in fact be aligned. In some embodiments howeverthe center aligned PWM is symmetric in that the start and end delaysbetween the pulses are equivalent. The pulse width modulation may befixed or variable frequency.

The controlled impedances may comprise either voltage controlledimpedances, or current controlled impedances. They may comprisetransistors, any of the following kinds of transistors may be used:insulated gate field effect transistor, IGFET, MOSFET, IGBT, RCIGBT,IGCT, GTO, JFET, HEMT or diode or a combination of these devices, or anyother suitable power switching device. As will be appreciated in thecontext of the present disclosure, the controlled impedances maycomprise switches. The controlled impedances may comprise a plurality ofsuch impedances coupled in parallel.

It will also be appreciated that the term “H-Bridge” is used herein, butthis should not be taken to imply any particular geometric arrangement.It will be appreciated by the skilled addressee that term H-bridge isderived from the typical graphical representation of such a circuit in acircuit diagram. An H bridge may comprise four controlled impedanceswhich may be arranged two in each leg of bridge as described above.These legs may be symmetric in the sense that the controlled impedanceseach have similar characteristics, for example the electricalcharacteristics of each leg of the bridge may be the same, and withineach leg the controlled impedances that couple the mid-point of the legto the upper and lower supply rails respectively may also be the same.

Embodiments of the disclosure are adapted to modify torque output in ashort pitched SR motor (either to reduce torque ripple, or to increasemaximum torque output). This has been described above with reference tocontrol of current reversals in the phase windings of the motor tomanipulate the mutual inductance terms set out in Equation 1. Oneexample of an embodiment which aims to achieve this comprises a systemcomprising at least a first apparatus according to any of claims 1 to 11(set out below), and a second similar apparatus. The controllers (whichmay be provided by a common controller) are arranged to control thetiming of the currents in respective first phase and second phasewindings of a switched reluctance motor to drive the motor, for exampleby controlling the voltage provider of both the first apparatus and thesecond apparatus. For example, these embodiments of the disclosure maycomprise a short pitched switched reluctance motor control apparatus forcontrolling current in a first phase winding and a second phase windingof a short pitched switched reluctance motor. This apparatus comprises afirst voltage provider configured to be coupled to the first phasewinding and operable to selectively apply voltage to the first phasewinding to drive current in the first phase winding in a first directionand in a second direction opposite to the first direction; a secondvoltage provider configured to be coupled to the second phase windingand operable to selectively apply voltage to the second phase winding todrive current in the second phase winding in a first direction and in asecond direction opposite to the first direction; and a controllerconfigured to control the first voltage provider and the second voltageprovider to control the timing of the currents in the first phasewinding and the second phase winding to drive the motor, and configuredto adjust the torque output from the short pitched switched reluctancemotor by selecting the timing of reversals in the direction of currentin at least one of the first phase winding and the second phase winding.This adjustment of the torque output may be achieved, as noted above, bycontrolling the direction of current in different phase windings, andthereby controlling the sign of the mutual inductance contributions (thecross terms) in Equation 1.

It will be appreciated by the skilled addressee in the context of thepresent disclosure that where reference is made to “cycles” electricalcycles are intended, rather than mechanical cycles of the motor.

The controller 4 may comprise any digital logic, such as fieldprogrammable gate arrays, FPGA, application specific integratedcircuits, ASIC, a digital signal processor, DSP, or by any otherappropriate hardware. In addition, all of the methods described hereinmay be embodied as computer program products operable to programprogrammable motor 1 control apparatus to perform these methods. Thesecomputer program products may be carried on non-transitory computerreadable storage media and may be distributed as computer readable datacarriers, which may include signals transmitted over a network.

Any feature of any one of the examples disclosed herein may be combinedwith any selected features of any of the other examples describedherein. For example, features of methods may be implemented in suitablyconfigured hardware, and the configuration of the specific hardwaredescribed herein may be employed in methods implemented using otherhardware. In some examples the functionality of the controllersdescribed herein may be provided by a general purpose processor, whichmay be configured to perform a method according to any one of thosedescribed herein.

The invention claimed is:
 1. A short pitched switched reluctance motorcontrol apparatus comprising: a voltage provider comprising a firstcoupling and a second coupling configured to be coupled to a phasewinding of the switched reluctance motor for applying a voltage to drivecurrent in the winding between the first and second coupling; and acontroller configured to apply a first voltage pulse to the firstcoupling, and to apply a second voltage pulse to the second coupling,wherein the start of the second pulse is delayed with respect to thestart of the first pulse, and the end of the first pulse is delayed withrespect to the end of the second pulse in which the controller isconfigured to time the pulses so that the centre of the first pulse isaligned with the centre of the second pulse.
 2. The control apparatus ofclaim 1 wherein the voltage provider is operable to selectively applyvoltage to the winding to drive current in the phase winding in a firstdirection and in a second direction opposite to the first direction. 3.The apparatus of claim 2 in which the controller is configured tocontrol the voltage provider so that the current in the phase windingflows in the first direction for at least one cycle of the switchedreluctance motor and in the second direction for at least one cycle ofthe switched reluctance motor.
 4. The apparatus of claim 3 wherein thecontroller is configured to control the voltage provider so that thedirection of the current alternates in successive cycles of the switchedreluctance motor.
 5. A short pitched switched reluctance motor controlapparatus comprising: a voltage provider comprising a first coupling anda second coupling configured to be coupled to a phase winding of theswitched reluctance motor for applying a voltage to drive current in thewinding between the first and second coupling in which the firstcoupling is coupled to a first supply voltage and to a second supplyvoltage by controlled impedances; and a controller configured to apply afirst voltage pulse to the first coupling, and to apply a second voltagepulse to the second coupling, wherein the start of the second pulse isdelayed with respect to the start of the first pulse, and the end of thefirst pulse is delayed with respect to the end of the second pulsewherein the controller is operable to control the controlled impedancesto control the voltage at the first coupling.
 6. A short pitchedswitched reluctance motor control apparatus comprising: a voltageprovider comprising a first coupling and a second coupling configured tobe coupled to a phase winding of the switched reluctance motor forapplying a voltage to drive current in the winding between the first andsecond coupling in which the second coupling is coupled to a firstsupply voltage and to a second supply voltage by controlled impedances;and a controller configured to apply a first voltage pulse to the firstcoupling, and to apply a second voltage pulse to the second coupling,wherein the start of the second pulse is delayed with respect to thestart of the first pulse, and the end of the first pulse is delayed withrespect to the end of the second pulse, wherein the controller isoperable to control the controlled impedances to control the voltage atthe second coupling.
 7. The apparatus of claim 6 wherein the controlledimpedances are arranged to provide a symmetric bridge such as anH-bridge.
 8. The apparatus of claim 6 wherein the controller isconfigured to control the voltage provider to select the direction ofcurrent in the phase winding in successive cycles of the switchedreluctance motor so as to balance the thermal load placed on thecontrolled impedances.
 9. A short pitched switched reluctance motorcontrol apparatus comprising: a voltage provider comprising a firstcoupling and a second coupling configured to be coupled to a phasewinding of the switched reluctance motor for applying a voltage to drivecurrent in the winding between the first and second coupling; and acontroller configured to apply a first voltage pulse to the firstcoupling, and to apply a second voltage pulse to the second coupling,wherein the start of the second pulse is delayed with respect to thestart of the first pulse, and the end of the first pulse is delayed withrespect to the end of the second pulse; wherein the voltage providercomprises controlled impedances arranged to provide an H-bridge, forexample wherein the H-bridge is arranged to be electrically symmetricabout a phase winding coupled to the voltage provider.
 10. A ofcontrolling a short pitched switched reluctance motor, the methodcomprising: providing a first voltage pulse to a first coupling of aphase winding of the switched reluctance motor, providing, to a secondcoupling of the phase winding, a second voltage pulse, delayed withrespect to the start of the first pulse and wherein the end of the firstvoltage pulse is delayed with respect to the end of the second voltagepulse; and wherein the voltage pulses are timed so that the centre ofthe first pulse is aligned with the centre of the second pulse.
 11. Themethod of claim 10 comprising selecting the voltages of the firstvoltage pulse and the second voltage pulse so that current through thephase winding flows in a first direction for at least one cycle of theswitched reluctance motor before flowing in a second direction, oppositeto the first direction, for at least one cycle of the switchedreluctance motor.
 12. The method of claim 11 comprising alternating thedirection of the current in successive cycles of the switched reluctancemotor.
 13. A tangible non-transitory computer-readable media carryingprogram instructions configured to program a processor of an electricmotor controller to perform a method according to claim
 10. 14. A methodof controlling current in a phase winding of a short pitched switchedreluctance motor so that the current through the phase winding flows inthe first direction for at least one cycle of the switched reluctancemotor before flowing in the second direction for at least one cycle ofthe switched reluctance motor, further comprising alternating thedirection of the current between successive cycles of the motor, whereincontrolling the current comprises controlling a plurality of controlledimpedances arranged to provide a an H-bridge for controlling the phasewinding, the method further comprising selecting the alternation ofcurrent so as to balance thermal load placed on the controlledimpedances.
 15. A tangible non-transitory computer-readable mediacarrying program instructions configured to program a processor of anelectric motor controller to perform a method according to claim
 14. 16.A system comprising at least a first apparatus and a second apparatus,wherein the first and second apparatus each comprise a short pitchedswitched reluctance motor control apparatus comprising: a voltageprovider comprising a first coupling and a second coupling configured tobe coupled to a phase winding of the switched reluctance motor forapplying a voltage to drive current in the winding between the first andsecond coupling; and a controller configured to apply a first voltagepulse to the first coupling, and to apply a second voltage pulse to thesecond coupling, wherein the start of the second pulse is delayed withrespect to the start of the first pulse, and the end of the first pulseis delayed with respect to the end of the second pulse; wherein thecontrollers of the first apparatus and the second apparatus are arrangedto control the timing of the currents in respective first phase andsecond phase windings of a switched reluctance motor to drive the motor,for example wherein the controllers are provided by a common controlleradapted to control the voltage provider of both the first apparatus andthe second apparatus, and wherein the controllers are configured toadjust the torque output from a short pitched SR motor by selecting thetiming of a reversal in the direction of current in at least one of thefirst and second phase windings, wherein the torque adjustment isselected to reduce torque ripple and wherein the frequency of currentreversals is selected based on the frequency of the torque ripple,wherein the timing of the current reversals is selected so that thecontribution of the mutual inductance between the first and second phasewindings combine additively with the contribution of the self-inductanceto one of: (a) increase the torque output from the motor; and (b) reducetorque ripple.